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This slug is distinguished from similar species by the position of its respiratory pore toward the front of its mantle, the lack of a keel on its back, and a wrinkled surface. It varies in color from orange or red to brown or black. The fringe of its foot is red-striped and its tentacles are dark.[3] It is 7 to 14 centimeters long.[1] Its mucus is sticky.[4]

The eggs are about half a centimeter long and are whitish when new, turning yellow-brown.[4]

It is very similar to the black slug, A. ater. The two can only be distinguished with certainty by dissection or molecular analysis.[4] It has been treated as a subspecies of A. ater in the past.[5]